This study contains the hydrogeological and hydrogecohemical investigations carried out in Diyarbakır and surrounding area. Mardin formation, which is the oldest unit in the field, forms the reservoir of the oil and its production water. The overlying Gercüş formation is aquiclude, and Midyat, Germik, Şelmo formations and basalt unit are aquifers. There are a number of contaminant sources for both surface water and groudwater in the field. Particularly, reinjection of the production water, extracted from the Mardin formation, to the Midyat formation that consists of karstic limestone, instead of Mardin formation, (which is the reservoir of the production water) creates a high risk for the Diyarbakır City potable water. The hydrogeochemical properties and vulnerability to contamination of the aquifers in the study area are discussed.